1. Field of the Invention
The present invention relates to semiconductor modules comprised of a plurality of switching semiconductor elements such as insulated gate bipolar transistors (IGBT) connected in parallel and at least a free wheeling semiconductor element such as a free wheeling diode (FWD) which is reversely connected in parallel to the plurality of switching elements.
2. Description of the Related Art
Hybrid vehicles and electric vehicles are equipped with a high electric-power conversion device in order to convert electric power between AC and DC power. In general, a hybrid vehicle uses two or more distinct power sources such as an internal combustion engine and an electric motor to drive the vehicle. On the other hand, an electric vehicle uses an electric motor to propel the vehicle.
FIG. 7 is a circuit diagram of a conventional electric-power conversion device 74 having a plurality of semiconductor modules 9. As shown in FIG. 7, the electric-power conversion device 74 capable of performing the conversion of high electric-power is comprised of a booster converter 741 and an inverter 742 which are placed between a DC power source 401 and a three phase AC electric motor 402. When the booster converter 741 boosts DC power supplied from the DC power source 401, the inverter 742 converts DC power to AC power, and then supplies converted AC power to the three phase AC electric motor 402.
On the other hand, during regeneration cycle, the inverter 742 converts AC power regenerated by the three phase AC electric motor 402 to DC power. The booster converter 741 steps down the converted DC power and supplies the DC power to the DC power source 401.
As shown in FIG. 7, the inverter 742 is comprised of six arms. Each arm has semiconductor elements 92 and 93 which are comprised of an insulated gate bipolar transistor (IGBT) and the like. In the inverter 742, the two arms forming a pair are connected in series. Thus, the two arms connected in series form a pair. The upper side arm 721 in each pair is electrically connected to a positive (+) electrode terminal of the DC power source 401, and the bottom side arm 722 is electrically connected to a negative (−) electrode terminal of the DC power source 401. As shown in FIG. 7, the upper arm 721 and the bottom arm 722 connected in series form a DC circuit. Total three DC circuits are placed in the inverter 742. A connection node between the upper arm 721 and the bottom arm 722 in each pair (as each DC circuit) is electrically connected to one of the U-phase electrode, the V-phase electrode, and the W-phase electrode of the three phase AC electric motor 402. That is, as shown in FIG. 7, the three connection nodes of the upper arms 721 and the bottom arms 722 in the three pairs are connected to the U-phase electrode, the V-phase electrode, and the W-phase electrode of the three phase AC electric motor 402, respectively.
The booster converter 741 is comprised of an upper arm 711 and a bottom arm 712 connected in series, and a reactor 413. The reactor 413 is connected between the connection node between the upper arm 711 and the bottom arm 712 and the positive (+) electrode terminal of the DC power source 401. Each arm in the booster converter 741 and the inverter 742 is comprised of the switching semiconductor element 92 and the free wheeling semiconductor element 93. The switching semiconductor element 92 and the free wheeling semiconductor element 93 are connected in parallel.
FIG. 8 is a perspective view of the conventional semiconductor module 9 to be placed in the conventional electric-power conversion device shown in FIG. 7.
As shown in FIG. 8, each arm is comprised of a single semiconductor module 9 in which the switching semiconductor element 92 and the free wheeling semiconductor element 93 are assembled together in a single module. For example, Japanese patent laid open publication No. JP 2001-308237 has disclosed such a conventional semiconductor module 9 which forms one arm.
However, because the built-in electric motor in vehicles such as hybrid vehicles need a large current, such a large current flows in each arm of the electric-power conversion device 74 incorporated in the vehicles.
FIG. 9 is a circuit diagram showing another structure of the semiconductor modules placed in the conventional electric-power conversion device. FIG. 10 is a plan view of another structure of the conventional semiconductor modules placed in the conventional electric-power conversion device.
As shown in FIG. 9, each of upper and bottom arms 911 and 912 in a booster converter 941 and upper and bottom arms 921 and 922 in an inverter 942 is comprised of a plurality of the semiconductor modules 9 connected in parallel in order to divide a large current.
However, as shown in FIG. 10, each arm is comprised of a plurality of the semiconductor modules 9 connected in parallel. This structure shown in FIG. 10 increases the total number of the semiconductor modules 9. This structure causes an increase in the entire size of the electric-power conversion device 94. That is, for example, when each arm is comprised of two semiconductor modules 9, the total number of the semiconductor modules 9 in the electric-power conversion device 94 becomes doubled. Because the electric-power conversion device 94 is comprised of the semiconductor modules 9 and cooling pipes 5, and those are laminated toward its lamination direction shown in FIG. 10, the total size of the electric-power conversion device 94 becomes doubled in area observed from its lamination direction.
In order to avoid such a conventional problem, there is a possibility of forming a single semiconductor module comprised of the two semiconductor modules 9. That is, a plurality of the switching semiconductor elements 92 connected in parallel and a plurality of the free wheeling semiconductor elements 93 connected in parallel are assembled together in a single semiconductor module.
However, when the switching semiconductor elements 92 are placed close to each other in the semiconductor modules 9, the current simultaneously flows in the switching semiconductor elements 92, and heat energy is simultaneously generated in the switching semiconductor elements 92. The heat energy rapidly increases the temperature of the semiconductor modules 9.
In order to avoid such a conventional problem, Japanese patent laid open publication No. JP H11-103012 has disclosed a unique structure to alternately place the switching semiconductor elements 92 and the free wheeling semiconductor elements 93 in the semiconductor modules so that the switching semiconductor elements 92 are not adjacent to each other.
In general, during the power running operation of the electric power conversion device 94 in which DC power is converted to AC power, the current mainly flows in the switching semiconductor elements 92. On the other hand, during the electric power regeneration operation of the electric power conversion device in which AC power is converted to DC power, the current mainly flows through the free wheeling semiconductor elements 93.
Further, the total heat energy generated in the switching semiconductor elements 92 during the power running operation is in general larger than that in the free wheeling semiconductor elements 93 during the electric power regeneration operation. During the power running operation, a current of a certain amount flows in the switching semiconductor elements 92. Still further, during the power running operation, the amount of current flowing in the switching semiconductor elements 92 tends to be larger than that in the free wheeling semiconductor elements 93. That is, it may be said that the switching semiconductor elements 92 generate a large amount of heat energy rather than the free wheeling semiconductor elements 93 not only in the power running operation but also in the electric power regeneration operation.
From such a point of view, it is preferable that the switching semiconductor elements 92 are placed at both end parts of the semiconductor modules 9, namely, the switching semiconductor element 92 is not placed between the free wheeling semiconductor elements 93 in order to eliminate the influence of the generated heat energy.
On the other hand, in the structure disclosed in Japanese patent laid open publication No. JP H11-103012, the free wheeling semiconductor element 93 is placed at one end of the arrangement of the switching semiconductor element 92 and the free wheeling semiconductor element 93. The switching semiconductor element 92 is placed between the free wheeling semiconductor elements 93. It cannot be always said that this structure disclosed in JP H11-103012 adequately radiates the heat energy generated in the switching semiconductor elements 92.